582 



TEXT-BOOK OF PHYSIOLOGY. 



traction of the radiating fibers of the iris, which collectively constitute the 

 dilatator pupilla muscle. With the entrance of light into the eye, the pupil 

 diminishes in size, in consequence of the contraction of the sphincter pupilla 

 caused by a stimulation of the peripheral ends of the pupillary fibers of the 

 retina, the degree of contraction depending within limits on the intensity 

 of the light. 



The action of the sphincter pupillae muscle is therefore a reflex action 

 and involves the usual mechanism, viz.: A receptive surface, the retina; 



afferent nerves, the pupil- 

 lary fibers in the optic 

 nerve; an emissive center, 

 the sphincter nucleus of 

 the motor oculi center ; effer- 

 ent nerves, including fibers 

 in the trunk of the motor 

 oculi and in the ciliary 

 nerves; and a responsive 

 organ, the muscle. (See 

 Fig. 270). That this is the 

 mechanism involved in this 

 reflex, is shown by the fact 

 that when any portion of 

 it is destroyed, the reflex 

 contractions of the sphinc- 

 ter are impaired or abol- 

 ished. 



As stated in a preceding 

 paragraph the central ter- 

 mination of the afferent 

 pupillary fibers concerned 

 in this reflex is not posi- 

 tively known. No one has 

 as yet succeeded in tracing 

 these fibers directly to the 

 sphincter nucleus. Experi- 

 mental and pathologic data 

 apparently disprove the 

 probability of their ter- 

 minating in the superior 

 corpora quadrigemina. It 

 has been shown, however, 

 that as the optic tract ap- 

 proaches its termination 



llan/nben 

 in Optic Tract 

 Direct,.^ 

 Crossed.. 



Transect ion of Spin ul Cord 



FIG. 270. DIAGRAM DESIGNED TO SHOW THE MECHANISM 

 OF THE IRIS REFLEX. The central termination of the 

 pupillary fibers is hypothetical. 



the visual and the pupillary fibers separate and it has been assumed that 

 the latter come into anatomic relation with some intercalated system which 

 in turn is connected with the sphincter nucleus. As to the situation, origin 

 and course of this system nothing positively is known. There is some evi- 

 dence for the view that these two systems are associated by commissural 

 fibers. 



